Surface gratings on VCSELS for polarization pinning

ABSTRACT

Methods for manufacturing a polarization pinned vertical cavity surface emitting laser (VCSEL). Steps include growing a lower mirror on a substrate; growing an active region on the lower mirror; growing an upper mirror on the active region; depositing a grating layer on the upper mirror; and etching a grating into the grating layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/299,638, “Surface Gratings on VCSELS For Polarization Pinning,” filedDec. 12, 2005, which is the U.S. Pat. No. 8,000,374, which claimspriority to U.S. Provisional Applications 60/673,219 titled “IntegratedVCSEL and Photodiode with Asymmetries for Polarization Control” filedApr. 20, 2005 and 60/711,311 titled “Amorphous Silicon Gratings forPolarization Control” filed Aug. 25, 2005, which are incorporated hereinby reference in their entireties.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The invention generally relates to polarization control in VCSELs. Morespecifically, the invention relates to using gratings for polarizationcontrol.

2. Description of the Related Art

Lasers have become useful devices with applications ranging from simplelaser pointers that output a laser beam for directing attention, tohigh-speed modulated lasers useful for transmitting high-speed digitaldata over long distances. Several different types of lasers exist andfind usefulness in applications at the present time. One type of laseris the edge emitter laser which is formed by cleaving a diode from asemiconductor wafer. Cleaving a diode from a semiconductor wafer formsmirrors that form a laser cavity defined by the edges of the laserdiode. Edge emitter lasers may be designed to emit a laser beam morestrongly from one of the edges than the other edges. However, some laserenergy will be emitted at the other edges. Edge emitter lasers arecommonly used when high optical power is needed.

A second type of commonly used laser is known as a vertical cavitysurface emitting laser (VCSEL). A VCSEL is formed in part by forming afirst mirror from Distributed Bragg Reflector (DBR) semiconductorlayers. The DBR layers alternate high and low refractive indices so asto create the mirror effect. An active layer is then formed on the firstmirror. A second mirror is formed on the active layer using more DBRsemiconductor layers. Thus the VCSEL laser cavity is defined by upperand lower minors which causes a laser beam to be emitted from thesurface of the laser.

One challenge that exists with the VCSELs mentioned above relates topolarization of optical beams. For example, in communication circuits,if polarized light is emitted from a laser device, the light can berouted using various types of beam splitters and polarization selectivefilters. However, often polarization in a VCSEL will change from batchto batch and depending on the operating conditions under which the VCSELis operating. For example, a VCSEL may have one polarization at a givenbias current and another polarization at a different bias current. Insensor applications, it is often important to emit a constantpolarization because part of the sensing operation relates to detectingdifferences in polarization. Thus, it would be advantageous toeffectively pin polarization in VCSELs.

BRIEF SUMMARY OF THE INVENTION

One embodiment described herein includes a method for manufacturing apolarization pinned vertical cavity surface emitting laser (VCSEL).

The method includes growing a lower mirror on a substrate. An activeregion is grown on the lower minor. An upper minor is grown on theactive region. A grating layer is deposited on the upper mirror. Agrating is etched into the grating layer.

Advantageously, the embodiments described above allow for polarizationpinned VCSELs to be manufactured. This allows the VCSELs to be used incommunication and sensor circuits where un-polarized VCSELs werepreviously not available for use.

These and other features of the present invention will become more fullyapparent from the following description and appended claims, or may belearned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand features of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 illustrates a cutaway view of a VCSEL and grating;

FIG. 2 illustrates a detailed view of a VCSEL grating;

FIG. 3 illustrates another detailed view of a VCSEL grating; and

FIG. 4 illustrates yet another detailed view of a VCSEL grating.

DETAILED DESCRIPTION OF THE INVENTION

Light from optical components can be described as polarized orun-polarized. Un-polarized light can more accurately be characterized aslight of a random polarization. Light may be emitted randomly from adevice, such as a VCSEL, in any one of an infinite number ofpolarizations. The polarization can be characterized by X and Y vectorsor by a magnitude and angle describing the direction of the electricfield, generally perpendicular to the direction of propagation, i.e. Z.In some cases, the light may be circularly polarized which may becharacterized by changing X and Y values as the light propagates. Iflight can be facilitated in one direction, while it is inhibited in anorthogonal direction, polarized light can be produced where all, or atleast for the most part, light is emitted in a single known polarizationdirection. On Zinc Blende substrates such as GaAs on or about 100orientation, there are two polarization directions, namely along the(011) and (01-1) directions. These are normally along and perpendicularrespectively to the major flat. Using asymmetries, light emissions canbe facilitated in one direction while they are inhibited in theorthogonal direction. Exploiting asymmetries and gratings may be used topin polarization in a VCSEL.

Referring now to FIG. 1, a cutaway view of a VCSEL structure isillustrated. The VCSEL 100 is formed from an epitaxial structure. Theepitaxial structure may include, for example, various layers formedthrough a process such as molecular beam epitaxy (MBE) or otherappropriate method. The example shown in FIG. 1 illustrates a GaAssubstrate 102. The VCSEL 100 includes a lasing cavity formed by a lowermirror 104 and an upper mirror 106. A lower mirror 104 is grown on thesubstrate 102. The lower minor 104, in this example includes alternatinglayers of higher and lower index of refraction materials. Each interfacebetween higher and lower index of refraction materials causesreflection. Thus by using an appropriate number of alternating layers, agiven reflectivity can be achieved.

An upper minor 106 is grown on an active region 108. The upper mirror106 is similar to the lower mirror 104 in that it generally comprises anumber of layers that alternate between a high index of refraction and alower index of refraction. Generally, the upper mirror 106 has fewerminor periods of alternating high index and low index of refractionlayers, to enhance light emission from the top of the VCSEL 100.

Between the mirrors 104, 106 is an active region 108 that includesquantum wells. The active region 108 forms a PN junction sandwichedbetween the lower mirror 104 and an upper mirror 106, which are dopedsuch that they are of opposite conductivity types (i.e. a p-type mirrorand an n-type mirror). In alternative embodiments, the minors may beun-doped or the mirrors may be dielectric minors. Free carriers in theform of holes and electrons are injected into the quantum wells when thePN junction is forward biased by an electrical current. At asufficiently high bias current the injected minority carriers form apopulation inversion (i.e. a higher concentration of free carriers inthe conduction band than electrons in the valance band) in the quantumwells that produces optical gain. Optical gain occurs when photons inthe active region cause electrons to move from the conduction band tothe valance band which produces additional photons. When the opticalgain is equal to the loss in the two mirrors, laser oscillation occurs.The free carrier electrons in the conduction band quantum well arestimulated by photons to recombine with free carrier holes in thevalence band quantum well. This process results in the stimulatedemission of photons, i.e. coherent light.

The active region 108 may also include or be formed near an oxideaperture 110 formed using one or more oxide layers 112. The oxideaperture 110 serves both to form an optical cavity and to direct thebias current through the central region of the cavity that is formed. Toform the oxide aperture 110, a trench 114 is etched down to the oxidelayer 112 to expose the oxide layer 112. An oxide 116 is then grown bysubjecting the exposed oxide layer 112 to various chemicals andconditions so as to cause portions of the oxide layer 112 to oxidize.The oxidized portion 116 becomes optically lower in index of refractionand electrically insulating so as to form the boundaries of the oxideaperture 110.

As illustrated in FIG. 1, a grating 122 may be included in a gratinglayer 124 on the VCSEL 100 to control polarization. The grating 122 maybe formed over the oxide aperture 110 so as to provide the polarizingeffect to light emitted from the oxide aperture 110. In the exampleshown, a low index of refraction layer 126 is formed through adeposition process on a high index of refraction layer of the top minor106. A high index of refraction layer 128 is formed through a depositionprocess on the low index of refraction layer 126. The grating 122 isthen formed into the high index of refraction layer 128 or alternativelyinto both the high index of refraction layer 128 and a portion of thelow index of refraction layer 126. In one embodiment where the grating122 is formed only into the high index of refraction layer 128, the lowindex of refraction layer 126 may be used as an etch stop layer foretching the grating 122 into the high index of refraction layer 128.

In one embodiment, the high index of refraction layer 128 and low indexof refraction layer 126 are thin layers as thin layers seem to exhibitbetter polarization and optical transmission characteristics. The highindex of refraction layer 128 may be for example Si or Si₃N₄. The lowindex of refraction layer 126 may be for example SiO₂. The index ofrefraction for the low index of refraction layer 126 may be for examplebetween 1.2 and 2.5. The index of refraction for the high index ofrefraction layer 128 may be for example between 1.8 and 5. In any case,the high index of refraction layer 128 has, in one embodiment, an indexof refraction that is 0.3 or greater than the index of refraction forthe low index of refraction layer 126.

The gratings 122 are designed to maximize the reflection of the desiredpolarization and to minimize the reflectivity of the perpendicularpolarization so the VCSEL 100 will lase in the polarization of highestreflectivity and be inhibited from lasing in the perpendiculardirection. VCSELs prefer to lase with polarizations in the (110) planes,so it may be advantageous to select one of the desired directions,although this is not required.

FIG. 2 illustrates a more detailed view of the gratings 122 used forpolarization control. FIG. 2 illustrates a grating 122 that includes anumber of protrusions 130. The protrusions 130 may be etched into thehigh index of refraction layer 128 which may be, for example, comprisedof Si or Si₃N₄.

To form the grating 122, the low index of refraction layer 126 and highindex of refraction layer 128 are deposited, as opposed to grown as istypically done with the minor layers in the top mirror 106 and bottommirror 104, on a high index of refraction layer of the top mirror 106 ofthe VCSEL 100 (FIG. 1). This can be done, for example, (PECVD (plasmaenhanced vapor deposition), by sputtering or evaporation at a reasonablylow substrate temperature. The low index of refraction layer 126 andhigh index of refraction layer 128 may be thin layers. The following isdescribed with thicknesses described in terms of optical thicknessessuch that one wave thickness is the thickness of a full wave in thatmaterial. For example silicon nitride has an index of refraction ofabout 2 at 850 nm. A wave of silicon nitride is therefore 850 nm/2.0 or425 nm. In some embodiments, the low index of refraction layer 126 mayhave a wave optical thickness of between 0.04 and 0.18. An opticalthickness of about 0.1 wavelengths for the low index of refraction layer126 functions well in one embodiment. The high index of refraction layer128 may be, in some embodiments, between 0.15 and 0.5 wavelengths thick.A thickness of about 0.25 wavelengths seems to function well in oneexemplary embodiment. The combined thickness of the low index ofrefraction layer 126 and high index of refraction layer may be, in oneembodiment, 0.25 waves thick. This may be done to create a lowreflectivity region outside the grating where it is not etched toprovide mode control. Examples of this are illustrated in U.S. Pat. No.5,940,422, titled “Laser With Improved Mode Control” which isincorporated herein by reference in its entirety.

The high index of refraction layer 128 is then etched using in oneembodiment direct write electron beam lithography. Direct write electronbeam lithography involves depositing an electron beam resist layer on asurface to be etched. An electron beam is then used to expose theresist. The electron beam can be controlled in much the same way aselectrons in a conventional cathode ray tube television are controlled.The electron beam is used to pattern a region that will be etchedleaving the protrusions 130. A reactive ion etch (RIE) or otherappropriate etch is then used to etch portions of the high index ofrefraction layer 128 to produce the grating 122. In one embodiment, astop etch may be used that selectively etches the high index ofrefraction layer, but stops on the low index of refraction layer 126. Inthis embodiment, the high index layer 128 may be Si₃N₄, and low indexlayer 126 may be SiO₂.

The protrusions 130 are formed periodically. For example, theprotrusions may be formed according to a period 132. The period may befor example in the rage of about 1 to 2 wavelengths of the laser asmeasured in air. This period allows for low cost photolithography whilemaintaining adequate discrimination between the two competing orthogonalpolarizations.

The protrusions 130 may also be etched according to a particular dutycycle. The duty cycle is the width 134 of the protrusions with respectto a period 132. The duty cycle should be less than 50% although above50% still functions. In particular, the duty cycle may be anywhere from10% to 60%. One embodiment functions particularly well with a 30% dutycycle.

Various alternatives embodiments will now be described in conjunctionwith FIG. 2. It may be preferable to use thin layers for the high indexof refraction layer 128 and low index of refraction layer 126. Forexample, in one embodiment illustrating an 850 nm VCSEL, the low indexof refraction layer 126 may comprise 70 nm of SiO₂. The high index ofrefraction layer 128 may comprise a 100 nm layer of Si₃N₄. The period132 of the protrusions 130 may be for example between 1 and 2 microns.It may be advantageous to maximize the period for ease of fabrication.While the period 132 is illustrated at 1 to 2 microns, other values mayalso be used. However, at about 750 nm, due to resonance, the lossfactor, a measure of loss of one polarization with respect to theorthogonal polarization, exhibited suggests that periods about thisvalue should be avoided. Even in the 1 to 2 micron range, anotherresonance is exhibited at about 1690 nm. However, the loss factor stillremains significant. At a 1690 nm period 132, there may be some loss ofreflectivity of the higher reflected polarization. This can becompensated for by adding more DBR periods in the top minor 106.

In the example shown in FIG. 2 where the VCSEL is an 850 nm VCSEL, theduty cycle of the protrusions 130 may be about 30%. In other words thewidth 134 of the protrusion 130 is 30% of the period 132. Other dutycycles may be used successfully, such as between 10% and 60%. Howeverbelow 50%, and at about 30% appears to be at or near the optimal dutycycle value.

For a 1280 nm VCSEL, the low index of refraction layer 126 may be 65 nmof SiO₂. The high index of refraction layer 128 may be 220 nm of Si₃N₄.The period 132 may be about 1.3 to 2.4 microns. In the 1280 nm case,resonance may occur at a 1280 nm period 132 and as such, periods ofabout this value should be avoided. The period 132 may be for examplebetween 1600 nm and 2300 nm. While other periods may be used, periodsbelow 850 nm become difficult to print using the etching process. Above2400 nm, the reflectance difference between the two orthogonalpolarizations weakens, making polarization more difficult to achieve.

In an alternative embodiment of an 850 nm VCSEL, the low index ofrefraction layer 126 may be 40 nm of SiO₂. The high index of refractionlayer 128 may be 75 nm of Si. In this example the period 132 is stillabout 1 to 2 microns and the duty cycle is about 30%.

In the example shown in FIG. 2 the low index of refraction layer 126 mayalso be used as passivation for the VCSEL 100 (FIG. 1). Typically, III-Vsemiconductors such as GaAs and AlGaAs can degrade when exposed to openair. By passivating the semiconductor materials with a layer such asSiO₂, the degradation can be slowed or prevented altogether. Thus, whenthe low index of refraction layer 126 is also used as an etch stop layerin the grating formation etch process, the low index of refraction layer126 may be used as passivation for the VCSEL 100 (FIG. 1).

Referring now to FIG. 3, an alternative embodiment of an 850 nm VCSELgrating 122 is illustrated. In the example shown in FIG. 3, the lowindex of refraction layer 126 is about 70 nm of SiO₂ and the high indexof refraction layer 128 is about 100 nm of Si₃N₄. In this exampleetching of the protrusions 130 is performed using a timed etch systemwhere the etchant etches into both the high index of refraction layer128 and low index of refraction layer 126. In the embodiment shown inFIG. 3, etching is done such that about 50 nm of the low index ofrefraction layer 126 remains on the VCSEL top minor 106. This 50 nmlayer may function as passivation protection for the VCSEL top minor106. As with the examples previously illustrated herein, the period isabout 1 to 2 microns. Also, the duty cycle, or the width of theprotrusion 134 as compared to the period 132, is about 30%.

Referring now to FIG. 4, an embodiment is illustrated where the grating122 uses silicon, such as amorphous or polycrystalline silicon. In thisexample, silicon (Si) is used as the high index of refraction layer 128on a lower index of refraction layer 126, which may be for example, SiO₂or of Si₃N₄. Si has a much higher index of refraction than Si₃N₄. Theindex of refraction for Si may be 3.2 to 4.3 depending on deposition andwavelength. As such, Si gratings can be thinner than other types ofgratings. Further, an increased difference in index of refraction byusing silicon increases the reflective difference between the twocompeting polarizations. FIG. 4 further illustrates a using a SiO₂overlay 136. The overlay 136 may be for example a ¼λ. In the exampleshown, the Si high index of refraction layer 128 is 50 nm. The SiO₂ lowindex of refraction layer is 40 nm. The period 132 is 1 to 2 microns.And the duty cycle is 30%.

Embodiments described herein can be further optimized to promote singlemodedness of the VCSEL 100. When the grating 122 is former essentiallycircularly over the aperture 110, and when the grating layer 124 is at athickness to act like an anti-reflective (AR) coating outside of thecircular region of the grating 122, the fundamental mode is selected.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A method of manufacturing a vertical cavitysurface emitting laser (VCSEL), the method comprising: growing a lowermirror on a substrate; growing an active region on the lower mirror, theactive region having one or more quantum wells configured to generatelight by stimulated emission; growing an upper mirror on the activeregion; forming an electrical aperture in or near the active region;positioning a light transmissive grating layer on the upper mirror andabove the electrical aperture, the light transmissive grating layerincluding a low index of refraction layer on the upper mirror and a highindex of refraction layer on the low index of refraction layer, the highindex of refraction layer having an index of refraction that is at leastabout 0.3 greater than the index of refraction of the low index ofrefraction layer; and forming a grating that extends at least partiallyinto the high index of refraction layer, the grating configured to pinpolarization of light emitted from the active region.
 2. The method ofclaim 1, wherein forming the electrical aperture includes: forming anoxide layer in the active region; etching a trench through the uppermirror to the oxide layer; and exposing at least a portion of the oxidelayer such that the exposed portion of the oxide layer oxidizes therebycreating the electrical aperture.
 3. The method of claim 1, wherein thegrating is etched into the high index of refraction layer.
 4. The methodof claim 3, wherein the low index of refraction layer acts as astop-etch when etching the grating into the high index of refractionlayer.
 5. The method of claim 1, wherein the grating extends at leastpartially into both the high index of refraction layer and the low indexof refraction layer.
 6. The method of claim 1, wherein the high index ofrefraction layer includes at least one of Si or Si₃N₄.
 7. The method ofclaim 6, wherein the high index of refraction layer includes an about100 nm thick layer of Si₃N₄.
 8. The method of claim 1, wherein the lowindex of refraction layer includes SiO₂.
 9. The method of claim 8,wherein the high index of refraction layer comprises an about 70 nmthick layer of SiO₂.
 10. The method of claim 1, wherein the high indexof refraction layer has a wave optical thickness of between about 0.15and about 0.5.
 11. The method of claim 1, wherein the low index ofrefraction layer has a wave optical thickness of between about 0.04 andabout 0.18.
 12. The method of claim 1, wherein the grating includes aplurality of protrusions, the period between each protrusion beingbetween about 1 micron and about 2 microns.
 13. The method of claim 12,wherein the width of each protrusion being between about 10% and about60% of the period of the protrusions.
 14. A method of manufacturing avertical cavity surface emitting laser (VCSEL), the method comprising:growing a lower mirror on a substrate; growing an active region on thelower mirror, the active region having one or more quantum wellsconfigured to generate light by stimulated emission; growing an uppermirror on the active region; forming an electrical aperture in or nearthe active region; depositing a light transmissive grating layer on theupper mirror and above the electrical aperture; and etching a gratinginto the grating layer, the grating layer being configured to pinpolarization of light emitted from the active region, the gratingincluding a plurality of protrusions, each protrusion having a period ofbetween about 1 micron and about 2 microns, the width of each protrusionbeing between 10% and 60% of the period of the protrusions.
 15. Themethod of claim 14, wherein the light transmissive grating layerincludes: a low index of refraction layer on the upper mirror; and ahigh index of refraction layer on the low index of refraction layer, thehigh index of refraction layer having an index of refraction that atleast about 0.3 greater than the index of refraction of the low index ofrefraction layer.
 16. The method of claim 15, wherein the low index ofrefraction layer acts as a stop-etch when etching the grating into thehigh index of refraction layer.
 17. The method of claim 16, wherein thegrating is etched into both the high index of refraction layer and thelow index of refraction layer.
 18. A method of manufacturing a verticalcavity surface emitting laser (VCSEL), the method comprising: growing alower mirror on a substrate; growing an active region on the lowermirror, the active region having one or more quantum wells configured togenerate light by stimulated emission; forming an oxide layer in or nearthe active region; growing an upper mirror on the active region; etchinga trench through the upper mirror to the oxide layer; exposing at leasta portion of the oxide layer such that the exposed portion of the oxidelayer oxidizes to create an electrical aperture; depositing a low indexof refraction layer on the upper mirror; depositing a high index ofrefraction layer on the low index of refraction layer, the high index ofrefraction layer having an index of refraction that at least 0.3 greaterthan the index of refraction for the low index of refraction layer; andetching a grating above the electrical aperture, the grating extendingat least partially into the high index of refraction layer, the gratingconfigured to pin polarization of light emitted from the active region.